Rsync / ssh high cpu load

Jan Alphenaar jan.alphenaar at
Wed Feb 17 15:16:58 MST 2010

Hi again,

Still working on this one. When I rsync/ssh a big file to my server over the
local network (100Mbit/s) with the --bwlimit=1000 I only have a small CPU
load. SSH takes about 10% and rsync approximately 0-5%. In the end the
transfer speed usually is around the value I set in bwlimit.

When I transfer the same file to a remote machine, where the bandwidth is
limited to roughly 140KB/s and the bandwidth limit is set to something below
this value I get around the same values which I have on the local network,
SSH 10% and rsync somewhere between 0-5%.

If I change the bwlimit value to 150KB/s or higher the SSH load goes up to
70% and rsync to 30% fully consuming my CPU.

I think the problem is pretty clear now. Does anybody have an idea, what the
next step could be to the resolution of this ?



-----Oorspronkelijk bericht-----
Van: Jan Alphenaar [mailto:jan.alphenaar at] 
Verzonden: woensdag 17 februari 2010 12:17
Aan: 'rsync at'
Onderwerp: RE: Rsync / ssh high cpu load


In an attempt to narrow this down a bit I installed a RedHat machine with
openssh-5.3p1 and rsync-3.0.7 and did the same test as below.

Transferring data, both up and downstream, are not giving me any CPU load.

Although there is a small version difference in the rsync versions, I would
say that this is caused by the cygwin layer.

I will mention this in the cygwin list.

Still, if anybody has any ideas how to solve the issue below, please let me

Warm regards,


-----Oorspronkelijk bericht-----
Van: Jan Alphenaar [mailto:jan.alphenaar at] 
Verzonden: dinsdag 16 februari 2010 17:31
Aan: 'rsync at'
Onderwerp: Rsync / ssh high cpu load

Hi everybody,

I am using Cygwin rsync/ssh to synchronize my files from my home computer to
my server. I raised a question on the Cygwin list as well, but thought this
list would be appropriate too.

The issue is that the CPU on the sending side is fully allocated by
rsync/ssh (both taking 50%) during a file transfer. This is the command I

	rsync -e ssh * <user>@<server>:/data

After some more testing on this I figured out that rsync/ssh is only taking
the full cpu if the bandwidth allocated to the session exceeds the true
bandwidth available on the link.

For example, if the uplink is 128KB/s and I pass in the --bwlimit=24
parameter the cpu is around 0%. Even --bw-limit=126 does not take 100% cpu,
but --bwlimit=132 does. Note this is the first install, the /data directory
on the receiving side is empty.

Would this be because of packet loss on the link and packets have to be
retransmitted ? I will do a quick test tonight with rsync/ssh between two
linux boxes (without cygwin), see what that gives me.

Note that I am using these versions:

Cygwin: 1.7.1-1
ssh: OpenSSH_5.3p1
rsync: 3.0.6

Has anybody seen this before ?

Thanks in advance.



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